The present invention relates to deepwater platform foundations. More particularly, it relates to providing cathodic protection to a tension leg platform foundation anchored to the ocean floor through a plurality of piles.
As used herein, a "tension leg platform" or TLP refers to any buoyant structure tethered to the ocean floor through substantially vertical tendons tensioned to draw the buoyant structure below its normal floating draft. Various embodiments include a full scale TLP having full drilling facilities, a tension leg well platform ("TLWP") having only a scaled down "completion" rig, a tension leg well jacket ("TLWJ") designed to accept well operations from an auxiliary vessel, or any other tendon deploying variation.
Tendons connect the buoyant hull to a foundation system at the ocean floor and are tensioned to draw the buoyant hull below its normal floating draft. The tendons transmit this static load to the foundation system. Further, the tendons must transmit this static load while subject to additional loads which have significant cyclical components driven by environmental forces of wind, wave and current on the hull and tendons. The combined load is transmitted to the ocean floor through the foundation system.
The bottoms of the tendons are secured to a foundation system at tendon receiving load connections or tendon receptacles. In the traditional practice for a steel foundation, the foundation system is built around a foundation template and pinned to the ocean floor with a plurality of piles, each secured to the template through a pile sleeve and connected thereto, e.g., by swaging operations deforming the pile into annular grooves on the surrounding pile sleeve. The tendon receptacles are provided elsewhere on the template. The template is a therefore framework which serves to permanently interconnect the tendon receptacles and the pile sleeves.
The template is cathodically protected by a plurality of sacrificial anodes that are conveniently placed before deployment. Conventional cathodic protection electrically connects the structural elements to be protected with sacrificial anodes which bear the corrosive effects of the saltwater environment. So long as sufficient cathodic current passes between the anode and the structural elements, only the anode will corrode and the structural elements will remain substantially unscathed.
However, the tendon-to-receptacle, to-template (and over)-to pile sleeve, to-pile, to-ocean floor load path of the conventional template based foundation system is an inefficient load transfer scheme, exacerbates the fatigue response, and creates handling difficulties. An improved TLP foundation system has been recently proposed which provides a more direct load transfer path of tendon-to-tendon receptacle-to-pile-to-ocean floor. This system is more fully disclosed in pending patent application Ser. No. 08/236,294, for a Direct Tendon to Pile Connection, filed by E. Doyle et al on May 2, 1994, the complete disclosure of which is hereby incorporated by reference into the present application.
However, deployment of that improved TLP foundation has potential drawbacks in embodiments eliminating the template and placing the tendon receptacles within the piles. Since no template is available for placing anodes. It is then very important to provide the piles with cathodic protection, including, if possible, the load bearing elements presented in the tendon receptacle which restrain the tendon after installation. The bottom of the tendon may be provided with its own anodes, but this will not ordinarily efficiently pass cathodic current across load the bearing interface to protect the load lugs in the tendon receptacle. This is especially a problem when the direct tendon to pile connection involves driven piles because anodes are typically made from soft materials such as aluminum, zinc, and magnesium alloys and those placed before pile installation will not stand up to pile driving operations which would tend to loosen or knock off the anodes.
Absent appropriate cathodic protection, substantially greater quantities of structural steel must be used to accommodate the corrosive effects of seawater over the life of the platform.
Therefore there is a need to provide cathodic protection to the piles of such a foundation system after pile installation.